1. Field of the Invention
An object of the invention is a thermal printer. The field of the invention is that of thermal printers, and more particularly that of thermal printers contained in payment terminals for bank cards, for example. These are therefore thermal printers with a small printing width and a fixed print head. The aim of the invention is to ensure that the paper on which the printing is to be done will be supported homogeneously on the print head, and to achieve this with a device that is simpler and costs less to manufacture.
2. Description of the Prior Art
In the prior art, bank payment terminals are provided with a thermal print head having a width of about 58 mm, and comprising about 384 printing points. This amounts to a definition of 6.6 dots per millimeter. This print head is fixed to the frame of the printer by an elastic device that gives the print head mobility along one direction. The thermal paper is held against the print head by a paper feed roller. The feed roller is fixedly held to the cover of the paper-loading bin. Thus when this cover is opened to refill the printer with paper, the feed roller moves away from the print head. This leaves the user fully free to position the paper. Then, when the cover is closed again, the paper-feed roller presses the paper against the print head. In order to position the roller accurately with respect to the print head and, therefore ensure that the paper is held properly, the roller rests against a fixed stop on the frame of the printer.
FIG. 1 shows a print head 101 fixed to a frame 102 by an elastic link 103 that gives the head 101 mobility in a horizontal direction. A sheet of paper 104 is held against the head 101 by a roller 105. The roller 105 is supported on a stop 106 fixed to the frame 102. Furthermore, the roller 105 is mobile around a hinge 107.
The problem with this device is that, when it is being made, it accumulates numerous dimensional and tolerance values that must be adhered to in order to ensure that the roller 105 is accurately positioned. The dimensions relate to the frame, on the stop 106, the hinge 107 and the print head 101. Indeed, while the link 103 makes it possible to compensate for a slight defect in a horizontal positioning of the roll, it cannot compensate for a defect in the vertical positioning. The stop 106 therefore needs to be fixed precisely with respect to the frame 102. Similarly, the play of the hinge 107 should be zero. This is therefore a solution difficult to implement.
The invention resolves these problems firstly by fixedly joining the stop and the print head and, secondly by making the print head mobile in two directions. Thus, the print head corrects the positioning errors of the paper feed roller caused by poor machining or by borderline compliance with dimensional requirements, along a horizontal direction and a vertical direction.
An object of the invention therefore is a thermal printer comprising a frame, a print head, a paper feed roller, and a securing stop for the paper feed roller wherein the securing stop for the paper feed roller is rigidly fixed to the print head.